1. Field of the Invention
This invention relates generally to electronic device fabrication, and more particularly to a method employing a printed mask to form active device elements by a printing process such that the printed mask controls dimensions of the active device elements.
2. Description of the Prior Art
Digital lithography is a maturing technology designed to reduce the costs associated with photolithographic processes, used often in the fabrication of micro-electronic devices, integrated circuits, and related structures. Digital lithography directly deposits patterned material onto a substrate in place of the delicate and time-consuming photolithography processes used in conventional manufacturing processes. The printed pattern produced by digital lithography can either comprise actual device features (i.e., elements that will be incorporated into the final device or circuitry, such as the source, drain, and gate regions of thin film transistors, signal lines, opto-electronic device components, etc.) or it can be a mask for subsequent semiconductor processing (e.g., etch, implant, etc.) Importantly, unlike traditional lithographic systems, digital lithography systems avoid the cost and challenges associates with the use of reticles or masks.
Typically, digital lithography involves depositing a print material by moving a printhead and a substrate relative to one another along a single axis (the print travel axis). Print heads, and in particular, the arrangements of the ejectors incorporated in those print heads, are optimized for printing along this print travel axis. Printing takes place in a raster fashion, with the print head making printing passes across the substrate as the ejector(s) in the print head dispense individual droplets of print material onto the substrate. At the end of each printing pass, the print head (or substrate) makes a perpendicular shift relative to the print travel axis before beginning a new printing pass. Printing passes continue in this manner until the desired pattern has been fully printed onto the substrate.
Materials typically printed by digital lithographic systems include phase change material and solutions of polymers, colloidal suspensions, such suspensions of materials with desired electronic properties in a solvent or carrier. For example, U.S. Pat. Nos. 6,742,884 and 6,872,320 (each incorporated herein by reference) teach a system and process, respectively, for printing a phase change material onto a substrate for masking. According to these references, a suitable material, such as a stearyl erucamide wax, is maintained in liquid phase over an ink-jet style piezoelectric printhead, and selectively ejected on a droplet-by-droplet basis such that droplets of the wax are deposited in desired locations in a desired pattern on a layer formed over a substrate. The droplets exit the printhead in liquid form, then solidify after impacting the layer, hence the material is referred to as phase-change.
Once dispensed from an ejector, a print material droplet attaches itself to the substrate through a wetting action, then proceeds to solidify in place. In the case of printing phase-change materials, solidification occurs when the heated and liquefied printed droplet loses its thermal energy to the substrate and/or environment and reverts to a solid form. In the case of suspensions or solutions, after wetting to the substrate, the carrier most often either evaporates leaving the suspended or dissolved material on the substrate surface or the carrier hardens or cures. The print material may also simply consist of a low-molecular weight monomer which cross-links and therefore solidifies upon irradiation with actinic radiation such as UV light. The thermal conditions and physical properties of the print material and substrate, along with the ambient conditions and nature of the print material, determine the specific rate at which the deposited print material transforms from a liquid to a solid, and hence the height and profile of the solidified deposited material.
If two adjacent droplets are applied to the substrate within a time prior to the solidification of either or both droplets, the droplets may wet and coalesce together to form a single, continuous printed feature. Surface tension of the droplet material, temperature of the droplet at ejection, ambient temperature, and substrate temperature are key attributes for controlling the extent of droplet coalescence and lateral spreading of the coalesced material on the substrate surface. These attributes may be selected such that a desired feature size may be obtained.
However, one disadvantage of digital lithography is that since the printed material is deposited in liquid form, it tends to spread after deposition and prior to solidification. When printing devices close together, for example less that 20-40 μm (micrometers) edge-to-edge, the droplet spreading can result in device-to-device short circuit (e.g., when forming electronic devices such as transistors), or device-to-device cross contamination (e.g., when forming an array of biological test units). While digital lithography has been used to isolate adjacent structures, such as wells for color filter material in a color filter for a flat panel display and the like, heretofore no device designs have successfully included printed well structures in forming active electronic devices, forming structures over active device layers, nor forming structures to confine the deposition of active device material.